Method and apparatus for testing the hardness of materials

ABSTRACT

Method and apparatus for testing the hardness of materials comprising means for effecting contact between the test material and an indenter by moving the test material under a progressively changing force into contact with the indenter to apply a minor and major load thereby producing two indentations of differing depth in the test material. The permanent depth of the indentation produced by application of the major load is determined by measuring the difference between the position of the indenter with respect to the test material when the minor load is applied both before and after the major load is applied and removed.

I United States Patent [151 3,657,92 1 Lang [4 1 Apr. 25, 1972 1 [54]METHOD AND APPARATUS FOR 2,839,917 6/1958 Webster ..73/81 TESTING THEHARDNESS OF 3,012,827 12/1961 Goetz ..308/35 T IALS 3,030,744 4/1962Mueller ..51/225 MA ER 3,102,417 9/1963 Chambers ....73/81 [72]Inventor: Elliot R. Lang, Hamden, Conn. 3,457,839 7/1969 Mills ..92/34[73] Asslgnee: 2:512: 3? & Cable Company '7"" Primary Examiner-RichardC. Queisser Assistant Examiner-Ellis J. Koch 1 [22] Filed: Dec. 31, 1969Attorney-Bennie, Edmonds, Morton, Taylor and Adams [21] Appl. No.:889,764 [57] ABSTRACT Method and apparatus for testing the hardness ofmaterials 2% comprising means for effecting contact between the test 1material and an indenter by moving the test material under a [58] FieldofSearch ..73/83, 81,82,84, 92/44, 43,

progressively changing force into contact with the mdenter to 92/34,308/DIG. l apply a minor and H1310! load thereby producing twoindentations of differing depth in the test material. The permanent 56]References cued depth of the indentation produced by application of themajor UNITED STATES PATENTS load is determined by measuring thedifference between the position of the indenter with respect to the testmaterial when the minor load is applied both before and after the majorload 6 is a plied and removed. 2,498,136 2/1950 p 2,628,495 2/1953 4Claims, 3 Drawing Figures 27 64 A 4 l I i A 2 STORING COMPARATER 5 9 26G5} I :z:

READOUT 8 E INDICATOR so PATENTEUAPR 25 I972 3, 6 57', 921 SHEEI 10F 2INVENTOR. ELLIOT R. LANG 2 BY wW MJ 44% A TORNEYS mmnmpn 25 new 3,657.921

SHEET 2 BF 2 STORING COMPARATER READOUT INDICATOR EXHAUST INVENTOR.

ELLIOT R. LANG ATTORNEYS BACKGROUND OF THE INVENTION The hardness ofmetal and other material can be determined by measuring the depth ofpermanent indentation produced in the surface of the test material bythe application of an indenter to that surface under a given appliedload which is commonly referred to as the major load. To obtain thismeasurement a double indentation method is commonly employed. Under thismethod, a reference point is first established by effecting contactbetween the indenter and the test material first under a minor load toproduce a slight indentation in the surface of the test material andthen under a majorload of known magnitude greater than the minor load tofurther indent the surface of the test material. Next, the differencebetween the depth of the indentations produced by the major and minorload is measured. This measurement is expressed in terms of a standardunit of hardness measurement, such as a Rockwell number, to obtain ahardness measurement of the test material.

In hardness testing equipment which utilizes the double in dentationtechnique, the minor and major loads are generally applied by separatemeans acting independently of each other. For instance, in a Rockwelltester, the means for applying the minor load comprises a mechanicalscrew acting on the test material to move it into penetrating engagementwith indenter while the means for applying the major load comprises aloading arm assembly which is moved into contact with the indenter undera force produced by falling dead weights. Due to the fact that separatemeans are employed for applying the major and minor load, extra time isrequired to complete the loading and unloading cycle. Typically 3 to 6seconds are consumed for this purpose. If higher speeds are attempted,the

. weights start to swing and the loading becomes inconsistent.

This, in turn, results in inaccurate hardness measurements.

Because of limitations on the speed of the testing cycle, the.

conventional Rockwell testeris not 'particularlysuitable for testing thehardness of successively fed items as, for example, items coming off anassembly line manufacturing operation and requiring quality controlchecking of their hardness.

The loading arm assembly of the Rockwell tester includes a loading armand a series of dead weights which are operatively connected to theloading arm via a suitable linkage assembly in such a manner that uponrelease of the weights the loading arm is caused to undergocorresponding downward movement into forceful engagement with theindenter under the force of the major load. The rate of downwardmovement of the loading arm is generally controlled by a suitable dashpot arrange ment. Despite the controlled nature of the descent of theloading arm, the force under which the indenter is brought to bearagainst the surface of the testmaterial is apt to build up too rapidlyproducing what is commonly referred to as shock loading of the testmaterial. Shock loading and, in particular, variations in shock loadingcan, for reasons well known in the art, be a significant cause ofinaccuracy in indentation hardness testing. Theuse of a dash pot in aRockwell tester effectively confines the degree of shock loading withinacceptable limits.

In other hardness testers utilizing a double indentationtechnique, themeans for applying the minor load comprises, like the Rockwell tester, amechanical screw acting on the test -material in one direction while themeans for applying the major load comprises an elaborate andcomplexbellows arrangement acting on the indenter in the opposite direction. Inaddition to being costly and complex, such constructions also requireextra time to complete the loading and unloadingcycle.

Conventional hardness testers which utilize a double indentationtechnique for measuring the hardness of materials require that theindenter and the test material be supported for relative movement withrespect to each other. For this purpose various types of mechanicalmeans have been devised for guiding the indenter and'test material intocontact position in tively connected to the air bearing assembly and,when acproper alignment for making acceptable indentations. Forinstance, various combinations of springs, mechanical bearings,elaborate frames and housing structures, and other alignment controllingdevices have been provided for this purpose. These constructions createerrors in measurement due to friction, bearing play and other conditionsinherent in the operation of such constructions.

SUMMARY OF THE INVENTION In accordance with the teachings of the presentinvention a high speed apparatus and method is provided for measuringthe hardness of materials fed rapidly through the apparatus. Broadly,the apparatus of the present invention is of the type wherein thepermanent depth of indentation produced in the test material under amajor load is measured to obtain a reading of the hardness of the testmaterial. In construction, the apparatus generally includes a framestructure, an indenter mounted on the frame structure, a support memberalso mounted on the frame structure for supporting the test materia],and a drive for moving the support with the test material thereonagainst the indenter under the minor andmajor loads. An air bearingassembly is provided for slidably mounting the support member on theframe structure for movement toward the indenter substantially free offrictional forces so that the depth of the indentation produced for agiven applied load will be a function solely of that load.

The drive for moving the test material into contact with the indenterunder the minor and major loads is provided by pneumatic means in thefonn of a bellows. The bellows is operativated, it acts in a singledirection to apply the minor and major loads in the proper sequence tothe support member and the test material resting thereon to bring thetest material into penetrating engagement with the indenter. Compressed'gas is supplied to the bellows at a progressively changing pres- ,surefrom an automatically regulated'supply source to produce the minor andmajor load in quick succession. The use of a pneumatically operatedbellows for applying the minor and major load insures that these loadsdo not build up to peak value instantaneously and produce damaging shockloads on the test material.

A filler block is positioned within the bellows to reduce the volumeofthe pressure chamber defined by the bellows so that the pressure buildsup quickly enough to allow the testing apparatus to be used for highspeed testing of numerous test specimens.

The hardness testing apparatus of this invention also in- I cludes meansfor measuring the depth of the indentation produced in the test materialunder the major load. This means is controlled by the regulated airsupply controlling the bellows drive and operates such that deflectionof the test material caused by spring compression under the major loadis allowed to occur before a depth measurement is taken. Inconstruction, this means includes an electromagnetic position transducerwhich is operatively connected to the test material. The transducerproduces an output signal indicative of the position of the testmaterial with respect to the fixed indenter. Two selected positionsignals from the transducer are fed into a storing comparator comprisingselectively operably circuit means for producing a single output signalwhich is indicative of the difference between the two selected positionsignals. One of the selected position signals from the transducer isindicative of the position of the test material with respect to theindenter when the minor load is applied. The other of these selectedoutput signals is also indicative of the position of the test materialwith respect to the indenter when the minor load is applied but afterthe major load has been applied and removed. The output signal from thestoring comparator representing the difference between these twoposition signals is therefore indicative of the depth of permanentindentation produced under the major load. This output signal isdisplayed by a conventional readout indicator.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is an elevation view, partlybroken away, of the hardness testing apparatus of the present inventionshowing the mechanical features thereof.

FIG. 2 is a cross-sectional view of the hardness testing apparatus ofthis invention taken along the lines 22 of FIG. 1 and showing also adiagrammatic representation of the selectively operable measuring means.

FIG. 3 is a schematic view of the regulator means for controlling thepressure of compressed air supplied to the bellows and the selectivelyoperable measuring means of the present invention.

DETAILED DESCRIPTION OF THE INVENTION As shown in'FIG. 1, the testingapparatus of the present invention includes a frame structure indicatedgenerally at 10, comprising a base member 1 and two threaded supportcolumns 2 upstanding from either side of the base member. An indenter 3made of. diamond or other suitable material is mounted in holder 4which, in turn, is attached to frame piece 5 by bolt means6 in themanner shown in FIG. 1. The frame piece 5 extends horizontally betweensupport columns 2 and the support columns extend longitudinally throughreceiving holes 7 disposed in the terminal ends of the support piece forthis purpose. The support piece 5 is supported in this position bythreaded retainer nuts 8 tightly engaging the upper and lower surface 11and 12 respectively of the frame piece. Although the indenter is held ina fixed position during operation of the testing apparatus, the abovedescribed construction of the structure supporting the indenteradvantageously permits the indenter to be moved to various verticalpositions simply by adjusting retainer nuts 8 the purpose of which willbe described more fully below.

The test material 20 is supported below the indenter 3 by anvil 13. Thetest material 20 is shown in FIG. 1 in its pre-test position wherein theupper surface of the test material is spaced apart from the tip of theindenter.

For the purpose of causing the upper surface of the test material to beindented so that hardness measurements can be obtained, drive means areprovided for effecting contact between the indenter and test materialunder the minor and major loads to produce two indentations of differingdepth in the test material. This means comprises an air bearingassembly, indicated generally by reference numeral 14, for

slidably mounting the anvil on the frame structure so that it may bemoved toward the stationary indenter and pneumatic means in the form ofa bellows 15 coacting with the air bearing assembly 14 for progressivelymoving the anvil in an upward direction and the test material supportedthereon into penetrating engagement with the indenter under the minorand major loads.

As shown most clearly in FIG. 2, the air bearing assembly 14 includes abearing housing 16 having a precision bore 17 and a bearing slide 18extending longitudinally through the bore 17. The housing 16 is attachedto bearing support 19 extending horizontally between the columns 2 at apoint located below the indenter. Bearing support 19 is held in a fixedvertical position by retainer nuts 21 in the same manner as frame piece5 described above in connection with FIG. 1. Like the frame piece 5 thebearing support 19 may be conveniently moved to the desired verticalposition simply by adjusting retainer nuts 21.

The bearing slide 18 is supported within the bore 17 for substantiallyfriction-free movement ina longitudinal direction by a relatively stift"curtain of pressurized gas surrounding the bearing slide and actingagainst the interior surface of the bore 17 and the exterior surface ofthe bearing slide 18. Pressurized gas is supplied to the space betweenthe bore and the bearing slide by a plurality of gas jets 22symetrically disposed about the bore. The gas jets 22 are, in turn,supplied with compressed gas via supply duct 23 and conduit 24 from anexternal supply source (not shown).

The upper end of the bearing slide 18 is connected by bolt 26 to cappingplate 27 which supports a cylindrical seating member 28. A removablearbor 29 having a reduced diameter portion 30 is disposed within theseating cylinder so that the shoulder 31 is in contact with the upperend of the seating member 28. The anvil has a shaft extension 32 whichfits within the centrally disposed bore 33 in arbor 29.

The lower end of the bearing slide 18 is connected by bolt 34 to bottomthrust plate 35 which in turn rests on pressure flange 36 of bellows 15.The thrust plate 35 has three evenly spaced pods 37 depending downwardlyfrom the under surface of the thrust plate to provide a tilt-freeuniform support for the bearing slide 18. The bearing slide is uniformlysupported by the pods 37 in the sense that its mass is evenlydistributedon the pressure flange 36 of bellows 15. Due to the stiffness of the airbearing supporting the bearing slide 18 in the housing 16 and due to theeven distribution of the weight of the bearing slide on the pressureflange, the test material is provided with substantially wobble-freemovement as it is being advanced toward the indenter under the drivingforce of bellows 15, all of which will be described in more detailbelow.

The bellows is connected to the upper surface of base member 1 by boltmeans 38 extending through the bottom flange 39 of the bellows. The airpressure chamber 41 defined by the bellows 15 is sealed in airtightrelationship with the base member 1 by O-ring 40. Pressurized gas issupplied to chamber 41 by a supply duct 42 and conduit 43 from anautomatically regulated supply of compressed air (not shown).

A filler block 45 is disposed within the bellows l5 and connect'ed tothe base member 1 by bolt means 46. The filler block 45 is constructedto reduce the volume of the gas chamber 41 defined by the bellows sothat pressure within this chamber may build up in a relatively rapidmanner but not rapidly enough to produce abrupt movement of the testmaterial into contact with the indenter. This feature advantageouslyaids in reducing the length of the loading and unloading cycle.

The bellows acts to move the test material into penetrating engagementwith indenter under a progressively changing force to sequentially.apply the minor and major loads. The progressively changing force on thetest material is provided by controlling theair pressure in bellows 15.For this purpose,

I the pressure of the gas supplied to the bellows is automaticallycontrolled by regulator means, reference numeral 50 in FIG. 3.

The regulator means comprises a regulator 51 communicating in serieswith shut-ofi valve 52 and a regulator 53 communicating in series with ashut-off valve 54. As shown, regulator 51 and shut-off valve 52communicate in parallel with regulator 53 and shut-off valve 54. Theregulator means also includes a flow control valve 56 connected inseries with regulator 51 and shut-off valve 52. Switch mechanisms 55 and56 are timed for operating the various valves 52, 54 and 56'. Also,switch mechanisms 56 and 55 operate exhaust valve 57 and the exhaustbleed valve 58. Compressed gas is supplied to the regulator means from asupply source (not shown). The operation of the regulator means will bedescribed below in the portion of the specification wherein adescription of the operation of the hardness testing apparatus of thisinvention appears.

The testing apparatus of the present invention also includes means formeasuring the depth of the indentation produced in the test materialwhen it is applied against the stationary indenter under the major load.In the presently preferred embodiment of this invention, this meansincludes an electromagnetic position transducer 59 the output of whichis indicative of the position of the test material with respect to theindenter. The transducer is connected to the bearing support 19 bybracket 60 and front plate 61 in the manner shown in FIG. 2. The core 62of the transducer is attached to arm 63 which is operatively connectedto the test material via seating member 28. With this arrangement, theoutput voltage of the transducer at any given instant is indicative ofthe position of the indicated generally by arm which, in turn, isindicative of the position of the test material with respect to thefixed position indenter. It will be recognized, therefore, that avoltage value which represents the difference between the output voltageof the transducer at two different positions of the test material withrespect to the indenter is indicative of the difference in the depth ofthe indentations produced in the test material under the applied loadsat each of these two positions.

For the purpose of determining this voltage value, the output of thetransducer is fed into a storing comparator 64. The comparator comprisesselectively operable circuit means for storing a voltage signalindicative of the position of the test material under the minor load andthereafter comparing this voltage signal with an input voltage signalindicative of the position of the test material under the minor loadafter the major load has been applied and removed. In making thiscomparison the comparator produces single output signal indicative ofthe difference between the two positions of the test material which iseffectively a measurement of the depth of the permanent indentationproduced in the test material under the major load. The single outputsignal from the storing comparator is displayed by a conventionalread-out-indicator 65.

The storing comparator is activated by the switch mechanisms of thepressure regulator means in a manner which will now be described in thefollowingportion of the specification wherein the operation of thehardness testing apparatus of this invention is described.

In operation, a test specimen is first placed on the anvil. This may beaccomplished manually or by providing a suitable automatic feedingmechanism for this purpose. The initial position of the indenter isadjusted so that its tip is spaced from the upper surface of the testmaterial. In the event automatic feeding means is employed forpositioning test specimens of different thickness on the anvil in asequential fashion, the indenter is positioned so that the specimenhaving the greatest estimated thickness will be located in spacedrelationship from the tip of the indenter when in position on the anvil.When its proper vertical position is determined, the indenter is fixedin that position by tightening retainingnuts 8 in the manner describedabove. 7

Next, the minor and major load values are determined and regulators-51and 53 are set respectively to a pressure corresponding to these values.Shut-off valve 52 is initially open while shut-off valve 54,exhaustvalve 57 and exhaust bleed valve 58 are closed. In this condition,pressure gradually builds up in the bellows to move the testmaterialupward into forceful but relatively gentle penetrating contactwith the indenter. The rate of build-up of pressure in the bellows iscontrolled by the flow control valve 56'. When the pressurecorresponding to the minor load isreached a slight dwell period occursat which time shut-off valve 52 is closed and storing comparator 64 isactivated and deactivated by switch mechanism 55. The storing comparator64 reads the output voltage of the transducer 59 to establish areference voltage indicative of the position of the test material withrespect to the indenter under the force of the minor load. Thisreference voltage is, as described above, stored in the storingcomparator for comparison with another voltage signal at a later time.The storing comparator is activated and deactivated virtuallyinstantaneously.

Simultaneously with closing of valve 52, shut-off valve 54 is opened byswitch mechanism 56. With shut-01f valve 54 now opened the pressure inthe bellows continues to build up until the test material bears againstthe indenter under the force of the major load. At this point in thecycle, shutofi' valve 54 is closed and exhaust bleed valve 58 andshut-ofi valve 52 are opened by switch mechanism 55. With the exhaustbleed valve now in open position, the bellows chamber is depressurizedby bleeding through the shut-off valve 52 and regulator 51 until theminor load pressure is restablished. At this point the storingcomparator is again activated and deactivated by switch mechanism 55.Thereby, the storing comparator again reads the output voltage of thetransducer. This second voltage the minor load position of reading,which is indicative of the position of the test material with respect tothe indenter under the applied force of the minor load after the majorload has been applied and removed is compared to the stored referencevoltage. This produces a voltage signal representing the differencebetween the two readings; and this signal is fed into the read-outindicator to obtain a comparative measurement of hardness. After thereadings have been taken, exhaust valve 57 is opened by switch mechanism56 and the pressure in bellows chamber 41 is reduced to zero. The testmaterial is then removed and the process repeated for the purpose oftesting the hardness of other test specimens. The value displayed by theread-out indicator may be compared automatically with the valuepreviously determined for a control specimen of known hardness toquickly indicate whether the test material has a satisfactory hardnessmeeting predetermined quality standards.

Due to deflection resulting from spring compression the test materialassumes a final operative position with respect to the indenter when theminor load is reestablished which lies between the initial position ofthe test material under minor load application and the position of thetest material under major load application. This final positionrepresents the true depth of the indentation produced by application ofthe major load. Since the major load is removed before the depthmeasurement is taken at this final position, there are no errors in theultimate hardness measurement due to spring compression in either thetest material or the frame structure of the testing apparatus. In otherwords, reverse movement of the test material due to spring compressionis exhausted prior to measuring the final operative position of the testmaterial with respect to the indenter;

From the above description of the operation of the hardness testingapparatus of this invention, it will be recognized that the minor andmajor loads are applied gradually in the sense that they are appliedwithout producing shock loads. With the present invention, the minor andmajor loads are, despite the gentleness of their application, appliedquite rapidly'in the sense that very little time is consumed by theloading and unloading cycle. ln fact, with the apparatus of the presentinvention, the time cycle for applying the minor and major load valvesand taking a measurement of hardness of a test specimen is reduced by asmuch as percent over the time period required with presently availablehardness testing equipment utilizing double indentation techniques tomeasure hardness. This feature renders the testing apparatus of thepresent invention particularly attractive for use in high speed testingoperations.

In addition to measuring the hardness of the test material, theapparatus of the present invention is uniquely capable of obtainingcomparativev measurements of thickness of the material being tested. Forthis purpose, the output voltage signal from the transducer at the minorload position of the test material may be compared with the outputvoltage signal at the minor load position of a control specimen of knownthickness to thereby obtain a comparative measurement of the thicknessof the test material with respect to the thickness of the controlspecimen. The comparative measurement so obtained will be in the form ofa voltage signal representing the difference between the output voltagefrom the transducer at the test material and the voltage output from thetransducer at the minor load position of the control specimen. However,if desired, this voltage value may be converted into standard units oflinear measurement to obtain an absolute measurement of the thickness ofthe test material. While it is recognized that due to the possibledifferences in hardness between the test material and the controlspecimen, the position of the test material with respect to the indenterunder the applied force of the minor load is not a true linear functionof the thickness of the control specimen, nevertheless, a relativelyaccurate approximation of the test material thickness can be obtainedwith the apparatus of the present invention. For most applications wherethickness measurements of the test material are needed, exactmeasurements are not required. Of course, if the control specimen andthe test material have the same hardness values, the measurement ofthickness obtained will be precise.

lclaim:

1. Apparatus for testing the hardness of materials comprising:

a. a frame structure;

b. an indenter held in a fixed position with respect to said framestructure;

0. a support for a test material;

d. pneumatic means for progressively moving said support with the testmaterial thereon into penetrating engagement with the indenter under aprogressively changing force to apply a minor and major load producingtwo indentations of differing depth in the test material, said pneumaticmeans including conduit means for connecting said support to a source ofpneumatic pressure;

. signal generating means for producing a first signal indicative of theposition of the test material with respect to the indenter under a forceequal to the minor load and a second signal indicative of the positionof the test material relative to the indenter under a force equal to themajor load;

. regulator means for indicating a first pressure within said conduiturging the workpiece against the penetrator under said minor load and asecond pressure corresponding to said major load; and

g. switch means for actuating said signal generating means toproduce'said first signal when said regulator means indicates said firstpressure and to produce said second signal when said regulator meansindicates said second pressure.

2. Apparatus according to claim 1 wherein:

a. a bellows connecting said conduit means to said support.

3. Apparatus according to claim 1 wherein:

a. said conduit means includes first and second parallel branch conduitsconnected to said support;

b. said regulator means includes:

1. a first pressure regulator and shut-off valve in said first branchconduit, and

2. a second pressure regulator and shut-off valve in said second branchcircuit; and

c. said switch means includes:

1. a first switch for actuating said first shut-off valve to shut saidvalve off when said first pressure regulator indicates said firstpressure and to open said valve when said second pressure regulatorindicates said second pressure, and

2. a second switch for actuating said second shut-off valve to open saidvalve when said first pressure regulator indicates said first pressureand to close said valve when said second pressure regulator indicatessaid second pressure.

4. Apparatus according to claim 1 further comprising:

a. selectively operable circuit means adapted to receive said first andsecond signals and to produce a single output signal indicative of thedifference between said first and second signals.

1. Apparatus for testing the hardness of materials comprising: a. aframe structure; b. an indenter held in a fixed position with respect tosaid frame structure; c. a support for a test material; d. pneumaticmeans for progressively moving said support with the test materialthereon into penetrating engagement with the indenter under aprogressively changing force to apply a minor and major load producingtwo indentations of differing depth in the test material, said pneumaticmeans including conduit means for connecting said support to a source ofpneumatic pressure; e. signal generating means for producing a firstsignal indicative of the position of the test material with respect tothe indenter under a force equal to the minor load and a second signalindicative of the position of the test material relative to the indenterunder a force equal to the major load; f. regulator means for indicatinga first pressure within said conduit urging the workpiece against thepenetrator under said minor load and a second pressure corresponding tosaid major load; and g. switch means for actuating said signalgenerating means to produce said first siGnal when said regulator meansindicates said first pressure and to produce said second signal whensaid regulator means indicates said second pressure.
 2. Apparatusaccording to claim 1 wherein: a. a bellows connecting said conduit meansto said support.
 2. a second pressure regulator and shut-off valve insaid second branch circuit; and c. said switch means includes:
 2. asecond switch for actuating said second shut-off valve to open saidvalve when said first pressure regulator indicates said first pressureand to close said valve when said second pressure regulator indicatessaid second pressure.
 3. Apparatus according to claim 1 wherein: a. saidconduit means includes first and second parallel branch conduitsconnected to said support; b. said regulator means includes: 4.Apparatus according to claim 1 further comprising: a. selectivelyoperable circuit means adapted to receive said first and second signalsand to produce a single output signal indicative of the differencebetween said first and second signals.